multipilot

Transition levels and altitudes are something I am still trying to get a solid grasp on, so help me out here if I'm wrong.

For example, if the Trans. Level (QNE) is FL200 and the Trans. Alt (QNH) is 18,000. The difference between the two is the transition layer. The way I understand it, when descending through a transition layer you go to local altimeter setting at the transition level. So in this case we'd set local altimeter (QNH) when descending through FL200. If we were climbing through the transition layer, we'd set 29.92 (QNE) when climbing through 18,000 feet.

Am I correct?

FlyerJosh

Yes, but don't over think it... Unless the altimeter setting is WAAAAY off from standard (29.92), transition altitude and transition level are going to be very similar.

I simply set local or standard when the altimeter hits 18,000 (in either direction). The only time that you really need to pay close attention is when you are leveling off at either FL180 or 17,000'. In such a case, I'll generally set the proper altimeter (29.92 or local) about 1500-2000' before level off to ensure that "george" doesn't get confused.

Bloodhound

Most places (if not all) in the U.S., the TA and TL are the same, 18,000. But it varies in other countries. I don't have a whole bunch of int'l experience, mostly Mexico, but the two are different at just about every place there. Usually, TA is 18k but the TL is 18,500 or 19k.

JetJocF14

Remember TA are when your going UP. TL are when your going DOWN. Here in the good old USA it just happens to be the same at 18000. However, in Europe the TA can be as low as 4000. While the TL would be something like 6000. You change your altimeter setting when you get a clearance to pass through one of these layers. IE Your level at 3000 feet. Mufasa airline climb and maintain FL 100. You would then switch to 29.92 (actually 1013) passing 4000 feet the TA.. Where you need to be heads up is say flying in bound and your descending and approach tells you to level off at 6000. Assuming the TL is 6000 that day. Then you would still maintain 2992, just as if you were leveling off in the US at FL180. Most countries will give you the local QNH when they give you a clearance to decend BELOW the TL. If in doubt ask the controller if he wants you to level off at 6000 on QNH or QNE. The french are notorious for this........... Have a nice day....

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multipilot

Thanks everyone! I'm prepping for an XJT interview and that's a popular question. I know in the good ol' USA QNE and QNH are both at FL180 and 18,000', but does anyone know of a practical reason it's different in places like Mexico?

Bloodhound

My pure, non-fact based assumption was it had something to do with higher terrain. But that's as far as I thought about it.

EDC757

One good way to know what to set and when is that the controller must give you an altimeter setting when they give you a lower altitude below the TL and when assighned a flight level they will say climb to 'Flight Level" ---

Freight Dog

A handy way to remember the difference between Transition Altitude and Level:

Transition Altitude (The "A" points up, meaning you set your altimeters to 29.92 going through this altitude on the way up)

Transition LeVel (The "V" points down, meaning you set your altimeters to QNH (local altimeter setting) passing that flight level on the way down)

cac737

The transition layers exists to allow for variations in possible low QNH settings and may dictate that FL185 be the lowest level in the US for low pressure conditions and elsewhere for that matter. Now for you mainly domestic guys, you don't often see pressure levels around 29.42 except maybe just before a hurricane or TD shows up on the eastern coast, but it is actually common in Anchorage and in other parts of the world where you actually do encounter it. Besides altimetry, those very low pressure levels actually make a huge impact on takeoff performance as well...but i digress....

I want to also add in that while yes, in other countries the TL, TA can be less than 10,000 or other than 18,000.......there are in some other countries where there is no set levels and you will be advised of the current day's TA or TL by ATC.

For those of you that want to aspire to intl. long haul flying, I would advise to keep good habits about you when setting altimeter settings. IE....if climbing from 12,000 to FL300 in the US dont change over to QNE whenever you feel like it or vice-versa...yeah yeah i know, whats the big deal about changing it at 17,000 when you are climbing or maybe even 17,400.....but here's the problem. One day many years from now on the back side of the clock with a O dark thirty arrival after a long ocean crossing into somewhere in the world....after years of bad altimetry habits you'll "miss" that the TL is at FL140 and reset it near FL180, and be given a stop descend at Fl 150 (which happens quite often) with a low altimeter setting you can now see the problem.

Best of luck with the interview.

multipilot

Thanks, and thanks for the info!